The problem is simply one of current draw causing the voltage seen at the receiver to drop below a safe margin. We saw this years ago with the JR Super Servos where you could not run more than 3 in an airplane using a 4.8v battery. As the current went up, voltage went down and the use of 4 or more could cause the receiver to see an unsafe voltage under load.

The 8550 is designed as a car servo where current draw isn't an issue and users want high speed and power. Most r/c cars have only 1 or MAYBE 2 servos installed. This servo also only has a 7" lead on it fitting in better with the car requirements.

As I said, I will try to find out the electrical specs on the servos from Horizon.